Drilling fluids are used in deep-well-drilling operations to carry off the rock which has been detached by the drilling tools, to cool the rotary drill and to limit entry of fluids into the hole. In large depths, the formation pressure increases so that the drilling fluid must be weighted with materials having a high specific gravity. Known weighting materials are, e.g., baryte, quartz, iron oxides and lead oxides, zinc dust (U.S. Pat. No. 2,099,825). The Opened German Specification No. 2,362,712 teaches a process in which non-magnetic materials which contain iron oxide and have a specific gravity of at least 4.2 are produced in that iron ore is calcined at 927.degree. to 1121.degree. C under oxidizing conditions and the burned material is subsequently quenched. The resulting product is intended for use as a weighting agent in drilling fluids.
It is also known to use calcination products of sulfide ores as a weighting agent in drilling fluids. German Pat. Specification No. 2,139,952 teaches a process in which pyrite is recovered by flotation and subsequently calcined in a fluidized bed and the calcine is used in a specific particle size below 75 microns. The Soviet Union Pat. Specification No. 70,794 teaches a process in which weighting materials for drilling fluids are produced by burning calcined pyrites at 700.degree. to 800.degree. C and then cooling them. The magnetite pyrite which is thus formed is subsequently separated and ground for use as the weighting material.
In accordance with U.S. Pat. No. 2,276,075, magnetic materials in a particle size below 44 microns are used as weighting materials. These materials consist, e.g. of artificial magnetite, which has been produced by a calcination of non-magnetitic iron oxides under reducing conditions.
The advantages of the use of magnetic weighting agents are seen in that they can easily be separated by a magnetic separator from the drilling fluid which has been contaminated with drilling detritus.
In the process taught in U.S. Pat. No. 2,298,984, a weighting agent is made by burning calcined pyrite at temperatures of at least 980.degree. C in a preferably oxidizing atmosphere and in the presence of accelerators consisting, e.g. of alkali metal salts. This treatment serves to eliminate sulfur to a residual content of about 0.2%. In the last-mentioned prior publication, apparently no significance was attached to the magnetic properties of the weighting agent because it was stated that the ignition can be effected in a neutral or reducing or oxidizing atmosphere and presumably the product is the usual ferromagnetic oxide.
It has previously been believed, as a rule, that ferromagnetic weighting agents consisting of iron oxides, such as magnetite, are the most suitable weighting agents, presumably because they can be more easily separated from spent drilling fluids.
Where calcined pyrite has been proposed as weighting agent, the highly magnetically susceptible magnetite contained therein in general as a result of the production processes has been tolerated or even desired. It has, however, been recognized (U.S. Pat. No. 3,867,128) that magnetic properties are disadvantageous.
Various solid iron oxides have different magnetic susceptibilities H, namely
______________________________________ Magnetite up to 90,000,000 .times. 10.sup.-6 Magnetite pyrite up to 150,000 .times. 10.sup.-6 Hematite up to 13,000 .times. 10.sup.-6 Limonite up to 2,800 .times. 10.sup.-6 ______________________________________
It has been found that the use of drilling fluids which are weighted with ferromagnetic iron oxides or iron oxides having a substantial ferromagnetic content produce undesired results and disturbances in the inductive monitoring of the position of the drill pipe in the hole.